1. Field of the Invention
The present invention relates generally to the field of medical imaging and, more particularly, to attenuation correction of images.
2. Description of Related Art
A Positron Emission Tomography (PET)/Computed Tomography (CT) or a Single-Photon Emission Computed Tomography (SPECT)/CT scanner can be an important tool for tumor imaging. It can provide both anatomical and functional information in a single imaging session and also accurate registration of PET and CT data to improve the diagnosis of tumors. Additionally, the high-performance and high-throughput multi-slice CT has also replaced the transmission rod sources to obtain an attenuation map for quantification of PET emission data. The scan time using transmission rod sources is several minutes for only 15 cm of coverage, but this is reduced to only about 30 seconds of CT scan time for 100 cm of coverage using high-throughput multi-slice CT. A PET/CT scan, thus, typically takes less than 30 minutes instead of more than an hour as with a stand-alone PET scanner.
Applicants recognized that a potential issue has emerged from the mismatch of CT and PET data caused by the difference of temporal resolution between CT (<1 sec) and PET scans, however. In a PET/CT scan, for example, a helical CT (HCT) scan is normally performed before the PET scan to obtain the CT-based attenuation map for the attenuation correction (AC) of the PET data and provide anatomical information to improve localization of the PET images. Performing CT before PET allows the PET data to be reconstructed with AC before the patient leaves the scanner and ensures the integrity of data acquisition. To match the CT and the PET data, patients are asked during the HCT scan to either hold their breath at mid-expiration or take a shallow breath. Misalignment of the HCT and the PET data has been reported with both techniques and may become an issue in imaging the thorax, such as where the breath-hold state in HCT is different from the free-breathing state in PET. The end results are a mismatch of tumor location between the HCT and PET scans and an inaccurate quantification of tumors in PET images.
Commercial PET/CT and SPECT/CT scanners with high performance multi-slice CT have a deficiency in imaging the thorax and the abdomen over which the cardiac and respiratory motion may compromise the quantification and localization of the lesions imaged by PET/CT and SPECT/CT.